In order to hold a workpiece to be machined on or in a workpiece spindle of a machine tool during the machining, a workpiece gripping system is required with a power-operated clamping device rotating with the workpiece spindle. The most widely used are hydraulically actuated clamping devices in the form of a so-called chuck which is attached to the front end region of the workpiece spindle and is closed for gripping a workpiece and opened for releasing the workpiece by means of a hydraulically actuated clamping cylinder. The clamping cylinder has a cylinder housing which is securely connected to the workpiece spindle and in which a piston in operative connection with the chuck can be displaced back and forth in the direction of the spindle axis in that either one or other of the sides of the piston is acted upon with pressure oil or with another pressure medium. In principle, it is, however, also conceivable, of course, for the chuck to open due to the action of one or more springs and for the piston of the clamping cylinder to be acted upon with pressure oil only from one side in order to overcome the action of this spring or these springs and to close the chuck. The pressure medium, generally hydraulic oil, is supplied to the rotating clamping cylinder and thus to the cylinder chamber located on the one side of the piston or to the cylinder chambers located on both sides of the piston via a (hydraulic oil) supply device in the form of a so-called rotary supply; a rotary supply to clamping cylinders of a conventional construction has an outer, non-rotating housing portion annularly surrounding the spindle axis and having two oil channels as well as an inner housing portion which rotates with the workpiece spindle and thus with the cylinder housing of the clamping cylinder and likewise has two oil channels, is surrounded by the outer housing portion and, where applicable, is formed by the cylinder housing of the clamping cylinder, wherein the one oil channel of the inner housing portion communicates with the one cylinder chamber and the one oil channel of the outer housing portion, the other oil channel of the inner housing portion with the other cylinder chamber and the other oil channel of the outer housing portion. An oil pump feeds the hydraulic oil via an oil control device to the rotary supply and thus to the clamping cylinder with the pressure required for a secure gripping of the workpiece; generally, the clamping cylinder is also acted upon with pressure oil via the rotary supply and the oil control device in order to open the chuck again. All these procedures are, like the drive for the workpiece spindle, controlled by the machine tool control, which is normally designed as a computerized numerical control and with which monitoring devices and regulating devices of the oil control device also communicate in order to monitor and regulate the oil pressure.
On account of the often high rotational speeds of the workpiece spindle, in particular, of the workpiece spindles of lathes and the considerable machining forces acting on the workpieces, the workpiece gripping systems must meet the highest safety requirements: The oil pressure on the one side of the piston of the clamping cylinder causing the gripping of the workpiece in the clamping device of the workpiece spindle must be constantly monitored, and when the supply of the gripping system with the pressure medium fails or takes place with too low a pressure, a clamping force sufficient to securely hold the workpiece must be maintained over a period of time which is long enough for the machine control to be able to bring the rotating workpiece spindle to a stop with any certainty since, otherwise, a workpiece could be catapulted away from the workpiece spindle with a high kinetic energy.
A reliable maintenance of the required gripping force is achieved in the known gripping systems in that two releasable check valves are provided in the double-acting clamping cylinder (i.e. the piston of which can be acted upon with the pressure medium from both directions) rotating with the workpiece spindle, namely one check valve in each of the two flow paths extending in the cylinder housing and leading to a respective side of the piston so that when the supply of pressure medium fails a pressure set beforehand can be maintained in each of the two cylinder chambers. The gripping system, its control and the check valves are designed such that the clamping device may be opened only when the workpiece spindle is stationary, wherein that check valve which is located in the flow path responsible for the closing of the clamping device is released and opened by the pressure built up in the other flow path for the opening of the gripping device (the same applies for the reverse case).
Such releasable check valves are, however, not only relatively complicated but they also have considerable functional disadvantages: As a result of their construction they restrict the flow of the pressure medium to a much greater extent than a simple, conventional check valve which results in the clamping device being closable and openable only relatively slowly; in order to shorten the actuating time for the clamping device, known gripping systems have for each flow path of the clamping cylinder several such releasable check valves connected in parallel to one another in order to reduce the total flow rate resistance. Since, in the case of known gripping systems with releasable check valves, the latter may be released and opened only when the workpiece spindle is stationary, it is not possible to reduce the clamping pressure or the gripping forces exerted on the workpiece by the clamping device while the workpiece spindle is rotating when a fine machining of the workpiece with a lesser clamping force could take place after a coarse machining, during which the workpiece must be held with a high clamping force in view of the high machining forces exerted on the workpiece, in order to reduce the workpiece deformations caused by high clamping forces for the fine machining which requires a great precision. Finally, the known gripping systems with releasable check valves in the clamping cylinder make it impossible to continuously alter the pressure prevailing in the clamping cylinder on the one side of the piston as a function of the rotational speed of the workpiece spindle in order to compensate for the centrifugal forces acting, for example, on the clamping jaws of a chuck--such a compensation possibility would allow the clamping forces actually acting on the workpiece to be kept constant irrespective of the rotational speed of the workpiece spindle and the workpiece deformations caused by the clamping forces to be reduced in this way. As is apparent from the preceding explanations, a reduction in the clamping pressure and thus in the clamping forces may be brought about in the known gripping system as described only with a considerable expenditure of time since, first of all, the workpiece spindle must be stopped, then the gripping device opened and the latter finally closed again with a reduced clamping pressure; in this respect, the workpiece must also be prevented from falling out of the clamping device by way of a pressing mechanism. Such a procedure does, however, also entail the additional risk of the exact clamping position of the workpiece which is decisive for an exact final machining to become lost when the workpiece is displaced somewhat in relation to the clamping device when this is open.
Finally, the known, releasable check valves are also not absolutely reliable in their functioning: They are certainly tested for leaks when they are new but there is no practical possibility to carry out such testing during the working time of the machine tool. Consequently, it cannot be ruled out that the check valve will lose its tightness as a result of dirt or gum formation in the hydraulic oil customarily forming the pressure medium, and even the slightest leak leads to an immediate drop in the clamping pressure when the supply of pressure medium fails--in this connection it must be kept in mind that the known rotary supplies unavoidably leak to a quite considerable extent and so a leaky check valve leads to an immediate drop in the clamping pressure on account of these leakage losses.
FR-PS 2 675 230 describes a gripping system with releasable check valves in the clamping cylinder which does, however, make a reduction in the pressure prevailing in the clamping cylinder possible when the workpiece spindle is running. For this purpose, pressure valves are built in parallel to the releasable check valves of the clamping cylinder and they open at a minimum clamping pressure of, for example, 10 bars, remain open at a higher pressure and then each form a bypass to the associated check valve, wherein the bypass is designed such that it causes a strong restricting effect. In the case of a sudden loss of pressure to below the value of the minimum clamping pressure specified above, these pressure valves close quickly enough in order to at least more or less maintain the high pressure prevailing in the relevant cylinder chamber thanks to the restricting effect of the bypass line. In the case of a slow drop in pressure, the pressure in the relevant cylinder chamber can, however, sink as far as the specified minimum clamping pressure; such a slow drop in pressure may be desired to take place in order to reduce the clamping force; it can, however, also be caused by a functional error in the supply of pressure medium, e.g. as a result of a leakage occurring during operation in a hose connecting line, and since a differentiation between these two cases is not possible, the workpiece may be catapulted out of the clamping device during a functional error in the supply of pressure medium.
In the case of the clamping cylinders of a conventional construction as described above and provided with a so-called rotary supply (hydraulic oil supply device for supplying and carrying away hydraulic oil to or from the rotating cylinder housing of the clamping cylinder) the outer, non-rotating housing portion of the rotary supply accommodates at least two bearings which are arranged at a distance from one another in the direction of the spindle axis and with the aid of which an inner housing portion of the rotary supply, which rotates with the clamping cylinder housing, is mounted in its outer housing portion, and the outer housing portion of the rotary supply is prevented from rotating with it by means of stationary stops.
The oil control device, via which the oil inflow and outflow to or from the rotary supply is controlled, is connected to the rotary supply in the case of all the known constructions via two flexible hose lines and includes a pressure oil storage device chargeable by the oil pump mentioned above, a pressure regulating valve located between the storage device and the rotary supply for regulating the oil pressure prevailing at the clamping device, as well as a check valve located between pressure oil storage device and oil pump to prevent undesired losses in pressure (if the unavoidable leakage losses in the rotary supply are disregarded). Such an oil control device fed from an oil pump results, for example, from DE-42 37 901-C2.